The present invention relates in general to a current-to-voltage converter with controllable gain, hereinafter referred to as IVC. More particularly, the present invention relates to an IVC suitable for use in a finite impulse response digital-to-analog converter (hereinafter referred to as FIRDAC), as well as to a FIRDAC provided with such an IVC, and the present invention will be explained in the following for this particular example. It is, however, to be noted that the present invention is not limited to the use in a FIRDAC.
The finite impulse response principle for a digital-to-analog converter is known per se, and described for instance in U.S. Pat. No. 5,323,157. Generally speaking, a FIRDAC comprises a shift register with a large number of stages, typically more than hundred stages, which receives a bitstream input signal of one bit, i.e. a serial data stream with one bit amplitude resolution. Each of the stages of the shift register switches a dedicated current source ON or OFF. The currents thus generated by all of the stages of the shift register are added to generate an output current of the FIRDAC. Usually, the output current is applied to a current-to-voltage converter to generate an analog output voltage of the FIRDAC. Each stage of the FIRDAC produces an output current contributing to the overall output current of the FIRDAC. However, the stages of the FIRDAC do not all contribute in the same extent. In order to obtain a desired filter characteristic, each stage of the FIRDAC has an associated weighting coefficient, which is constituted by the magnitude of the output current of the current source.
In a typical application situation, the FIRDAC is used in a signal-processing path of a mobile telephone for providing an analog audio signal to a speaker or earphone.
Said dedicated current sources of the FIRDAC receive a reference current from a bias block, which comprises a stable reference voltage source and a bias resistor. Further, said current-to-voltage converter (IVC) of the FIRDAC comprises an operational amplifier and a feedback resistor. It is desirable that said bias resistor of the bias block and said feedback resistor of said IVC match very well, because the better the matching between said two resistors, the better the gain stability of the FIRDAC.
Further, it is desirable that the said feedback resistor of the IVC and said bias resistor of the bias block are built from the same type of resistors, because then the stability of the voltage output of the IVC is related to the stability of said reference voltage and the absolute value of the resistors is not important.
In principle, the above mentioned requirements are relatively easy to meet on one single chip by constructing each of said resistors as a series connection of a predetermined plurality of unit resistors which are mutually substantially identical.
However, it is further desirable that the gain of the IVC is programmable or controllable in steps of 3 dB. In designing a current-to-voltage converter, or an amplifier, with a controllable gain, it is known per se to use a feedback ladder resistor network comprising a plurality of resistors and a plurality of controllable switches, wherein the feedback resistor value is controlled by selecting a switch. However, in such a setup it is not possible to combine the requirement of substantially identical unit resistors with the requirement of gain steps of 3 dB. In previous designs, the requirement of using unit resistors has been dropped. In such previous design, the feedback resistor ladder comprises a series connection of resistors, at least one of those series resistors being a non-unit resistor. For obtaining a resistor device having a resistance value similar to a unit resistor, a further non-unit resistor is used, such that the combination of the resistor values of the first mentioned non-unit resistor and the further non-unit resistor is approximately equal to the resistance value of a unit resistor. As a consequence, for each selected switch a feedback loop is established which comprises at least two non-matching resistors.
The present invention aims to overcome or at least alleviate said problems. More particularly, it is an objective of the present invention to provide a current-to-voltage converter with a controllable gain, making use of unit resistors as much as possible in order to obtain improved matching with a bias resistor.